1. Field of the Invention
The present invention relates to personal computers, and more particularly to an apparatus and method of internally isolating personal computer components powered by an auxiliary power supply from components powered only by a main power supply.
2. Description of the Related Art
Recently, in certain personal computers an auxiliary power supply has been used to keep part of the system functional when a main computer power supply is down. The components in such a part of the system receiving power from the auxiliary power supply are used to "wake up" the machine at the appropriate time.
When the main power turns off, an auxiliary powered component with an output connected to an input of a component powered only by the main power supply should be prevented from driving a signal on that signal line to the "high" state. The auxiliary powered component must be prevented from driving a signal to the high state, because applying a voltage to the input of the component without power may supply enough voltage/current to cause the component to start operating randomly. Random operation is not desirable as randomly active signals can potentially disturb internal circuitry of the auxiliary powered components.
Further, inputs to auxiliary powered components connected to a component powered by the main power supply would float in voltage level in certain cases. This would occur, for example when the main power turns off if the inputs to the auxiliary powered components were not properly terminated. Many personal computer components are complementary metal oxide semiconductor (CMOS) integrated circuits. Leaving unused inputs of a CMOS component floating causes problems. A CMOS component with an input left floating may enter the active mode of operation with only minimal leakage current, due to the component's high input impedance. Normally, leakage currents are such that a CMOS component with an input floating will enter the active mode. Also, the unpowered input signal line can act like an antenna and begin to absorb signal energy present in the machine. After doing so, it can supply a valid logic voltage to the auxiliary powered component. This can disturb circuits not expecting such levels on that signal line. In addition, switching of the internal CMOS circuitry is inefficient as it needlessly consumes power.
One solution to this problem has been to couple an external pull-down resistor between ground and the signal line being isolated, or an external pull-up resistor between a positive rail of an active power supply and the signal line being isolated.
However, this approach had several undesirable features. The pull-up or pull-down resistors were separate or discrete circuit elements. If a large number of signal lines needed isolation, large amounts of board space could be consumed by the added resistor elements. Further, the additional resistors increased the computer system cost in terms of both the cost of the added resistors and the burden to the system of extra components.